.denglu{
    width: 500px;
    height: 400px;
    position: absolute;
    right: 0;
    left: 0;
    top: 0;
    bottom: 0;
    margin: auto;
  
}
html{
   width: 100%;
   height: 100%;
   background:url(../img/bg.jpg)no-repeat 100% 100%;
}
.formbox{
    width: 100%;
    height: 100%;
    position: absolute;
    right: 0;
    left: 0;
    top: 0;
    bottom: 0;
    margin: auto;
    background-color: rgba(0, 0, 0, .3);
}
button{
    float: right;
}
.register{
   margin-right: 20px;
}